1. Field of the Invention
This invention generally relates to digital signal transmission and processing, and specifically to a method and apparatus for combatting interference through the use of deterministic sequences, such as training patterns. It is particularly applicable for combatting co-channel and multipath interference for High Definition Television (HDTV) transmission.
2. Discussion of the Related Art
In "Method and Apparatus for Combating Co-channel NTSC Interference for Digital TV Transmission", U.S. Pat. No. 5,452,015, a precoding approach is described in an HDTV transmitter to combat the effects of co-channel NTSC interference. A technique for equalization based on precoding at the transmitter and filtering at the receiver is presented in "New Automatic Equalizer Employing Modulo Arithmetic", Electronic Letters, pages 138-139, March 1971, and in "Matched-Transmission Technique for Channels with Intersymbol Interference", IEEE Transactions on Communications, pages 774-780, August 1972, which are incorporated by reference herein. This approach will hereinafter be referred to as the Tomlinson-Harashima approach.
Co-channel interference can be expected to be located at particular frequencies, as compared to "white noise" which is characterized as being spread randomly across the frequency spectrum. To minimize the effect of co-channel interference, the receiver filter described in the '015 patent is designed to attenuate the received signal at select frequencies. In so doing, however, the receiver filter will also attenuate the desired signal at these frequencies, therein introducing signal degradation.
The transmitter-precoding receiver-filtering approach of Tomlinson-Harashima has been developed to allow for the attenuation of the interference which may be added to the desired signal, without attenuating the desired signal. Conceptually, such an effect can be accomplished by having the transmitter accentuate the signal at the frequencies which the receiver filter attenuates, so that the attenuation at the receiver produces the original, unaccentuated, signal. Unfortunately, a direct application of the inverse of the receiver at the transmitter is not practical. In the HDTV co-channel interference case, for example, accentuating the transmit signal at the co-channel frequencies would result in additional interference at that co-channel. Each TV channel would, in effect, increase their interference to other TV channels, which would be counter-productive to a scheme intended to combat such interference.
The Tomlinson-Harashima approach provides for an effective accentuation at the select frequencies without adding significant interference at these frequencies by employing a modulo M, non-linear, precoding of the signals. This non-linear precoding spreads the interference across the bandwidth, which appears to the receiver as random white noise, which has little effect on the overall receive quality at the cochannels.
As described in the '015 patent, the transmit-precoder employing the Tomlinson-Harashima approach comprises a modulo M reducer whose output is fed back through a filter with the inverse characteristics of the receiver filter, as shown in FIG. 1. Characteristically, the transmit-precoder can be described as a non-linear Infinite Impulse Response (IIR) filter whose feedback characteristics are the inverse of the feed forward characteristics of the Finite Impulse Response (FIR) filter at the receiver.
In "VSB Transmission System: Technical Details," Feb. 18, 1994, incorporated by reference herein, it was proposed that a periodic training sequence be utilized in HDTV transmission to enable equalization and minimize the effects of transmission channel distortions such as tilt and ghosts. The operation of the training sequence in conjunction with the precoder-filter approach is described in copending application Ser. No. 8/550,128, which issued on Feb. 11, 1997 as U.S. Pat. No. 5,602,602. The training sequence undergoes the same precoding as the data signals, and undergoes the same filtering at the receiver as the data signals. Since the training sequence is known, any differences between the received training sequence and the true training sequence is an indication of interference which has not been eliminated by the receive filter. Environmental factors, such as multipath reception caused by the reflection of the transmitted signal off a building, produce frequency dependent distortions. The observed difference between the true training sequence and the received training sequence is used to adjust the filter characteristics of the particular receiver to reduce such environment dependent interference. Similar environmental and location dependent interferences would exist with cable or other forms of data transmissions, for which this training sequence approach would also be effective.
Difficulties exist, however, with the combination of training sequences and the precoding-filtering approach. Ideally, the received training sequence should be directly comparable to a predefined true training sequence. This would allow for an unbiased determination of the actual difference between what was transmitted and what was received. Because the precoder at the transmitter is an IIR filter, which is characterized as having infinite memory, the precoded transmitted sequence will be different at each transmission, even though the same training sequence is used each time. The actual transmitted signal is a combination of the signal to be sent at this time plus every signal which was ever sent by the transmitter at prior times. The transmitted signal, and hence the received signal, is said to be non-deterministic, because of this infinite memory. Communication systems routinely determine the original, non-precoded, data without knowing the infinity of prior transmissions. This determination is complicated by the fact that the precoding is non-linear, but the fact that the transmitted information is not exactly the same as the original information is of no consequence for communications because other techniques exist for correcting errors. For receiver training, however, the training methods can only approximate the difference between the received and transmitted training sequence, because the actual transmitted data is unknown at the receiver. This approximation is effective for minimizing distortions which are not too severe and generally time-invariant, but the lack of a deterministic pattern precludes correction for distortions which are characterized by severe distortions or rapidly time varying phenomena. That is, direct comparisons of the differences from one training sequence to the next cannot be made, because the transmitted sequence is caused to be different each time by the IIR filter, and the effects of non-linear filtering cannot be distinguished from the effects of interference.